1. Field of the Invention
The invention is directed to a component working with acoustic bulk waves, particularly a bandpass filter or a duplexer having an electronic matching network.
2. Description of the Related Art
Components that work with acoustic bulk waves are bulk acoustic wave components (BAW) components that comprise individual thin-film resonators (also called thin film bulk acoustic wave resonator (FBAR)). In the future, these components will become of great significance as filters and duplexers in front-end modules of wireless communication systems.
A duplexer represents an electrical three-port structure that connects the antenna both to the transmission path as well as to the reception path of a terminal for mobile communication. A duplexer is composed of two bandpass filters to be connected to an antenna that are usually arranged in a common housing, in which one filter is located in the transmission path and another filter is located in the reception path.
On the one hand, a duplexer should comprise a good matching to the environment in each of the pass bands at the corresponding side of the three-port structure in order to minimize signal losses at the interface and, on the other hand, should assure a high suppression of the transmission signals in the reception path and, above all, of the transmission signals in the reception path. Another demand made of a duplexer is a high transmission power compatibility without deteriorating the filter function during a given service life of the component. These factors should all be balanced against minimizing the space requirement.
Up to now, for example, duplexers of microwave ceramic are known that comprise a high power compatibility given low manufacturing costs that, however, are relatively large compared, for instance, to duplexers constructed on the basis of surface acoustic wave (SAW) technology. Although the SAW duplexers are in turn small, they are not especially power-resistant, which is problematic especially in the frequency range above 1 GHz.
U.S. Pat. No. 6,262,637 discloses a BAW duplexer that is composed of two separate bandpass filters for the transmission and reception domain. Both filters are soldered on a printed circuit board and electrically connected to an additional circuit of discrete elements that is likewise applied on the printed circuit board, so that the overall structure can be considered to be a three-port structure. Each of these BAW filters is composed of a ladder-type arrangement of individual acoustic resonators. Each resonator contains a piezoelectric layer that has both sides provided with electrode layers in which a cavity for reflecting an acoustic wave back into the resonator is arranged between the multi-layer resonator structure and a carrier substrate lying below this.
These BAW components require an electrical matching network for matching to their circuit environment, as already indicated above. Such a matching network can comprise inductors, capacitors and delay lines and primarily serves for matching the impedance of a component to the outside environment.
It is known that a BAW component (for example, a duplexer constructed of BAW components) can be provided with a matching network arranged on a printed circuit board (see U.S. Pat. No. 6,262,637). Due to a high space requirement and due to high manufacturing costs, however, such an arrangement is disadvantageous compared to currently available duplexers. Moreover, the electrical properties of such a component are very sensitive with respect to the relative, mutual arrangement of the filter and matching elements required for the duplexer function that are arranged on the printed circuit board.
It is known that matching networks in BAW components are implemented as integrated networks in which the circuit elements forming the network are integrated together in a multi-layer substrate. For example, the integrated circuit elements can be a component part of a duplexer circuit.
It is known that a SAW component (chip) having a multi-layer carrier substrate (of, for example, ceramic) that comprises integrated matching elements can be secured and electrically connected using a flip-chip arrangement or by wire bonding. One disadvantage of SAW components is that they have a short service life given high transmission power applied in terminals of mobile communication. A further disadvantage of the SAW duplexers is that the edges of the pass bands do not drop off steeply enough in their transfer function, for example, which is why SAW duplexers are not considered for applications with a slight spacing between the center frequencies of the transmission and reception domains.